The present invention relates generally to the silicon metal powders and methods for producing such powders. In particular, this invention relates to methods of making silicon metal powders wherein the silicon particles have a spherical morphology. In another aspect, this invention relates to methods of controlling the surface chemistry of silicon metal powders.
Silicon metal is produced commercially by carbothermal reduction of silicon oxide in electric furnaces. The molten silicon is tapped from the furnaces and usually cast into the form of large blocks of silicon metal. Silicon powders are then produced by mechanically crushing and grinding the blocks to produce irregularly shaped particles. Silicon powders may also be made directly by atomizing the molten silicon metal tapped from the reduction furnaces as described in U.S. Pat. No. 5,128,116. Silicon metal powders are used in a number of different applications such as semiconductors, solar cells, rocket fuel, high purity alloys and nuclear fuel applications. Many of these applications require one or more of the following properties: high purity, high crystallinity, or a spherical particle morphology.
Spherical silicon particles have been made by a number of processes. For example, in one method, U.S. Pat. No. 4,637,855 describes the fabrication of silicon spheres by heating a slurry of metallurgical grade silicon on the surface of a high temperature substrate. The substrate and the silicon are heated beyond the melting point of silicon causing beads of silicon to appear. These beads form spheres due to the high cohesion of silicon. The spheres are cooled whereupon they crystallize and are removed from the substrate surface.
In another method, U.S. Pat. No. 5,069,740 describes making single crystal silicon spheres by oxidizing the surfaces of silicon particles and then melting the particles to cause impurities to enter into the silicon dioxide skin. The molten particles tend to become more spherical because of surface tension forces. The molten spheres are cooled and then etched with hydrofluoric acid to remove the silicon oxide skin and the impurities contained therein. The steps are then repeated to obtain the desired purity. An intermediate shotting step can be used to form uniform particle sizes.
In yet another method, U.S. Pat. No. 5,817,173, spherical silicon crystals are formed by melting the tips of projections formed integrally on an silicon crystal base.
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide a method for making substantially spherical silicon metal particles having a low oxygen content.
It is a further object of the invention to provide a method for spheridizing silicon metal particles at a high rate of throughput.
In accordance with one object the invention, there is provided a method for spheridizing silicon powders comprising:
(a) entraining silicon metal particles having an irregular morphology in a gas stream;
(b) injecting the entrained particles into a plasma reactor containing a reducing atmosphere;
(c) melting at least 50% by weight of the silicon metal particles to form molten droplets; and
(d) solidifying the droplets to form substantially spherical silicon metal particles.
In accordance with another object of the invention, the irregular silicon particles are injected into the plasma reactor at a rate of at least about 10 pounds per hour.
In accordance with another object of the invention, the method is capable of making substantially spherical silicon metal particles having an oxygen content less than about 1000 ppm.